The inventions of both this application and the aforesaid parent application relate to a powdered metal filter composition, and processes for producing the same for the preparation filter-packs particularly useful for the filtration of the liquid polymer in the manufacture of synthetic fibers. The filtration of the liquid polymers is well-known in the art as is more completely described in U.S. Pat. No. 3,896,028.
In the past, a number of different filter media have beem employed in the synthetic fiber art including silica sand, metal shavings and metal powders. Until the advent of the aforesaid inventions, silica sand was the preferred filter media.
Silica sand exhibits a number of disadvantages. It is extremely brittle with the result that fines of silica sand develop which tend to obstruct the orifices in the fiber spinnerette. Additionally, the specific surface area of silica sand is substantially less than that of atomized metal powder for any given filter pack volume. Still further, silica sand tends to break down or fracture under pressure in the filter bed rather than deforming thereby favoring the production of fines during use.
Metal shavings exhibit less filtration efficiency than either silica sand or metal powder because of the relative lack of surface irregularities. Moreover, they tend to deform excessively or break in use.
Previously, metal powder was sintered into a filter form for convenience of handling. However, this tended to reduce filtration efficiency by reducing the specific surface area available for filtration. Moreover, the process of pressing and sintering was extremely costly. This cost factor was compounded by the fact that the rapid loss of filtration efficiency by clogging of the interstices required reprocessing by burning out the polymeric sludge which in turn tended to further reduce filtration efficiency. The result was that the sintered metal powder filter frequently had to be discarded after as few as four burn-outs.
Atomized metal powder which is prepared under specific conditions exhibits an extremely irregular surface which tends to enhance its filtration efficiency. However, previous metal powder filters prepared from loose stainless steel powder lacked the mechanical strength under pressure to maintain their apparent density within those limits for optimum filtration efficiency. Hence, the previous resort to sintering the metal powder to achieve some degree of mechanical strength.
It is among the objects and advantages of the present invention to produce a powdered metal filter composition which exhibits both the apparent density and enhanced resistance to compressibility for optimum filtration efficiency with little or no particle deformation and collapse under operating pressures. These characteristics have been achieved by increasing the percent of silicon, manganese and carbon in the metal feed for the atomization process as well as by controlling the angle of intersection of the water atomization jets with the molten metal stream.
In the parent application, a composition including 10% to 15% nickel and 2.0% to 3.0% molybenum was disclosed. Since that time, the cost of both nickel and molybdenum has increased substantially. Therefore it is the further object of this invention to reduce the nickel and molybdenum content without adversely affecting the aforesaid desirable characteristics of the composition for use as a filter media.
The processes and compositions disclosed and claimed herein achieve the desiderata high mechanical strength without embrittlement with a minimum use of nickel and molybdenum.